Directional microphone for head mounted midget hearing aids

ABSTRACT

A miniature microphone for a head-mounted midget hearing aid wherein the diaphragm divides the microphone housing into two separate chambers, the housing having a forward portion with a sound admitting aperture therein which is substantially nonrestrictive to passage of sound waves therethrough so that the sound waves may freely enter one of the chambers within the housing; the microphone housing also having a wall portion at the rear of the microphone, said wall portion having a plurality of minute apertures therethrough and defining a transmission time delaying acoustic resistance admitting sound pressure variations into the other chamber of the housing.

United States Patent 1 [111 31,777,79 Fischer et a1. j 1 4, 1973 [54]DIRECTIONAL MICROPHONE FOR HEAD 3,310,628 3/1967 Cragg et a1. 179/1 DMMOUNTED MIDGET H N AIDS 3,261,915 7/1966 Gorike et al. 179/1 DM [75]Inventors: Detlet Fischer; Johannes G. A. Wittkowski, both of Hamburg,Germany [73] Assignee: Willco-Horgerate Med. Apparatebau G.m.b.H.,Hamburg, Germany Primary Examiner-Kathleen H. Clafi'y AssistantExaminer-Thomas L. Kundert AIt0meyH. Dale Palmatier [57] ABSTRACT [22]Filed: 1971 A miniature microphone for a head-mounted midget [21] Appl.No.: 191,404 hearing aid wherein the diaphragm divides the microphonehousing into two separate chambers, the housing having a forward portionwith a sound admitting [52] f CL 179/121 179/1 ap rture therein which issubstantially non-restrictive [51] Km Cl H04r 1/22 to passage of soundwaves therethrough so that the [58] Field H5 5 R sound waves may freelyenter one of the chambers within the housing; the microphone housingalso having a wall portion at the rear of the microphone, saidReferences. fited wall portion having a plurality of m nute aperturestherethrough and defining a transmission time delay- UNITED STATESPATENTS ing acoustic resistance admitting sound pressure varia-3,082,298 3/l963 Gorike l79/l2l D tions into the other chamber of thehousing 3,240,883 3/1966 Seeler 179/121 D 3,223,782 12/1965 Weingartner179/1 DM 1 Claim, 8 Drawing figures 179/178, 179, 180, 1 DM; 181/31 RTHEMED SEE 41973 SHEET 2 [IF 3 Fig.6

INVENTORS JO/MNNES 6/3. Mfr/(0mm WHMF F/smm TTQM/EV DIRECTIONALMICROPHONE FOR HEAD MOUNTED MIDGET HEARING AIDS BRIEF SUMMARY OF THEINVENTION This invention relates to a directional microphone forhead-mounted midget hearing aids comprising a casing divided by adiaphragm into two chambers, a front casing wall having an openingcommunicating with one of these chambers and adapted to admit soundwaves impinging from the front on the easing into the one chamher, and awall portion adjacent the rear casing wall having a transmission timedelaying acoustic resistance adapted to admit sound waves impinging fromthe rear on the easing into the other of these two chambers.

A directional microphone of this general type is known in the Germanspecification Ser. No. 1,277,347, which has been laid open topublication inspection. In this known directional microphone thetransmission time delaying acoustic resistance is formed by a multitudeof very narrow channels having relatively great lengths as compared totheir diameters. This results in a phase shift of about 180 degrees whenaddressingthe microphone from the front. When addressing the microphoneexactly'from the rear side thereof there occurs substantially norelative phase shift, since the acoustic resistance and associatedcavity will produce a delay substantially'equal to the time required forthe sound waves to cover a longer path up to the front face of thediaphragm. By making use of this phase shift there is obtained anamplification when addressing the microphone from the front side, and anattenuation when addressing the microphone from the rear side. In theprior art directional microphone, the transmission time delayingacoustic resistance formed by a multitude of narrow channels consists ofa block of capillary tubes assembled in a compound block. This blockwhich may be made of a ceramic material for example may be ratherdifficult to manufacture. Furthermore, this'block requires considerablespace, and space is very scarce in head-mounted midget heating aids.

In larger directional microphones which are for general use, not inmidget hearing aids, it is known to form the resistance at the rear sideof the casing by a felt or a fabric having suitable openings therein.Materials of this type are unsuitable in directional microphones formidget hearing aids since with such small dimensions the resistancecannot be consistently reproduced.

It is the object of the present invention to provide a directionalmicrophone of the above-indicated type that is of smaller dimensionsthan the prior art directional microphones for midget hearing aids, andwhich may be manufactured more economically and at less cost.

This object is achieved, in accordance with the present invention, inthat the transmission time delaying acoustic resistance is formed by aplurality of small apertures in a thin wall portion adjacent the rearcasing wall.

It has been found in tests that the number and the depth or length ofthe apertures in the wall portion'adjacent the rear casing wall formingthe acoustic resistance must have a predetermined relationship to eachother, for a given diameter of the apertures and a given air volume ofthe chamber disposed at the rear side of the diaphragm.

Thus the present invention is based upon the surprising discovery thatthe extended channels at the rear side of the diaphragm which are foundin the prior ceramic block may be'replaced by a smaller number ofapertures in that casing wall of the microphone which is remote from thefront opening and established communication with the other chamber atthe rear of the diaphragm. Merely the above-indicated relationship iscritical and must be maintained in a given microphone structure. If thestructure of the microphone, i.e., the outer dimensions of themicrophone as well as the front opening and the air volumes in the twochambers are fixed, the optimum resistance may always be attained withalike apertures. Therefore all microphones will be of an identicalstructure so that the desired reproducibility in the manufacturingprocess is achieved.

The directional microphone of the present invention is not only of asimple structure and may be economically manufactured but is alsoextremely small.

According to the present invention, the apertures forming the acousticresistance may be arranged in a discrete plate or panel which may bemade of a suitable material such as stainless steel, copper, nickel orother metals or plastics. The apertures or holes in the plate may beformed by any of a number of processes such as etching orelectroforming. Forming by etching may result in the holes having atapered rather than straight shape through the thickness of the plate orpanel. As far as could be established, the smallest opening size of theapertures ordinarily represents the critical and measured dimensions.

By suitable means such as a screw or an adjustable pin which may belocked in a certain position it is furthermore possible to influence theair volume in the chamber located at the rear side of the diaphragm, inorder to tune the microphone or adjust for optimum performance ofthedirectional microphone after manufacture.

BRIEF DESCRIPTION OF DRAWINGS In the following, the invention will beexplained more in detail with reference to several illustrative embodiments shown in the appended drawings wherein:

FIG. 1 is a cross-sectional view through a directional microphoneaccording to the present invention whereby the microphone is a ceramictype microphone;

FIG. 2 is an end elevational view of the directional microphone shown inFIG. 1;

FIG. 3 is a cross-sectional view through another embodiment of adirectional microphone according to the present inventionwherein themicrophone is a magshown in FIG. 5;

FIG. 7 is a cross-sectional view through a further embodiment of adirectional microphone according to the invention wherein'the microphonemay be a condenser or electrostatic microphone; and

FIG. 8 is an end elevational view of the directional microphone shown inFIG. 7.

DETAILED DESCRIPTION OF THE INVENTION The microphones shown in thesedrawings each comprise a casing or housing 1 which is divided by adiaphragm 2 having obverse and reverse sides into two chambers 11 and 12respectively disposed adjacent the obverse and reverse sides of thediaphragm. A partition wall serves to mount various components.

An opening 4 serves to admit sound waves received from the front andwithout restriction to the obverse chamber 12 which is disposed on theone side of the diaphragm 2. In the illustrated form, the rear wall ofthe casing has a large opening which is substantially closed by a thinplate or panel 7. Minute apertures 9 in the plate 7 serve to admit soundwaves received from the rear of the casing tothe reverse chamber 11 andform the transmission time delaying acoustic resistance of themicrophone.

In the embodiment shown in FIGS. 1, 2 and 5, 6, ceramic or respectivelypiezoelectric type microphones with a corresponding ceramic transducer 3are used. In the embodiment shown in FIGS. 3 and 4 a magnetic microphonewith a corresponding transducer is used. In the embodiment shown inFIGS. 1 and 2 there is provided an adjustable device 6 for varying theair volume in the reverse chamber 11. The device 6 may be an adjustableplunger to be moved in the direction of the arrow, or may be a screw.The device fits tightly in the casing I so that when originallypositioned, it will remain fixed in location. The same adjustment device6 may also be used in the microphones shown in FIGS. 3-6.

In the embodiment shown in FIGS. 7 and 8 a condensor microphone or anelectrostatic microphone is used. In these directional microphones thechambers 11 and 12 are likewise separated by a diaphragm 2. Connectingpassages 13 communicate the lower side of the diaphragm 2 with thechamber 11. These connecting passages 13 pass through a counterelectrode14. A spacer ring 15 serves to maintain a desired spacing to anamplifier board 16. The apertures 9 are here likewise formed in adiscrete plate 7.

In the various embodiments, the electrical connectors are generallydesignated by the reference numeral 8.

In the following, practical examples of microphone dimensions and sizesof etched apertures 9 will be indicated.

The microphone shown in FIGS. 1 and 2 was constructed with a length of8.4 mm, a width of 5.6 mm and a height of 4 mm. By these dimensions theair volume of the chambers 11 and 12 is substantially fixed.

An optimum performance was obtained in trials with three apertures 9having a diameter of 0.12 mm, and a thickness of the plate 7 of 0.06 mm;or with eighteen apertures 9 having a diameter of 0.070 mm and a platethickness of 0.07 mm.

The directional microphone shown in FIGS. 3 and 4 has a length of 8.6mm, a width of 5.5 mm and a height of 4.1 mm. An optimum performance wasobtained with nine apertures 9 having a diameter of 0.096 mm and with aplate thickness of 0.07 mm.

The still smaller microphone of the embodiment shown in FIGS. 5 and 6 inwhich the plate 7 with the apertures 9 is mounted at the bottom of thecasing 1 had a length of 8.4 mm, a width of 5.6 mm and a height of 2.4mm.

With the latter microphone an optimum performance in tests was obtainedfor fourteen apertures 9 having an aperture diameter of 0.06 mm and fora plate thickness of 0.07 mm; as well as for three apertures 9 having amean aperture diameter of 0.092 mm and with a plate thickness of 0.07mm.

The directional microphone shown in FIGS. 7 and 8 has a length and awidth of 7 mm and a height of 4 mm. This microphone is provided withnine apertures having a diameter of 0.062 mm, and the thickness of theplate is 0.07 mm.

In all of these examples, the very thin plates are similar to metalfoil. Because of the extreme thinness, and the tapered shape of theapertures due to formation by etching, the smallest dimension of eachaperture is believed to be the important consideration as contrastedwith the less significant material thickness or length of aperture.

It will be seen that we have provided a new and improved directionalmicrophone for head-mounted hearing aids wherein the transmission timedelaying acoustic resistance is provided by minute apertures or holes ina panel or plate forming a portion of the casing wall for admittingsound pressures to the chamber at the rear of the diaphragm.

We claim:

1. A directional microphone for head-mounted eyeglass and behind-the-earhearing aids, comprising:

a rigid metal casing of box-like configuration, the casing having frontand rear portions including rigid walls with exterior and interiorsurfaces and the walls defining front and rear access openings into thecasing, the front access opening in the wall of the front portionproviding substantially unrestricted sound access into the casing, therear access opening in the wall of the rear portion being many timeslarger than required to provide sound access into the casing,

a vibratable diaphragm in the casing and dividing the interior of thecasing into two chambers, said diaphragm being disposed between saidaccess openings and restricting air communication across the interior ofthe casing and between said two chambers, the diaphragm being coupled totransducer means in one of the chambers to produce electric signals inresponse to vibration of the diaphragm, and

a discrete plate larger than said rear access opening and constructed ofthin foillike metal in contrast to the rigid metal walls of the casing,said discrete plate overlying and obstructing the entire rear accessopening in the wall of the rear portion of the casing and continuouslysecured around the entire periphery of the plate to the exterior surfaceof the casing, said discrete plate having a multiplicity of minuteapertures therethrough, each of said apertures having substantially thesame size as adjacent apertures and each of said apertures providingopen communication from the exterior of the casing and directly andlinearly through the rear access opening and into the adjoining chamberwithin the casing, and said multiplicity of minute aperturescumulatively defining the sole acoustic resistance coordinated with thevolume of the chamber between the discrete foil plate and the diaphragmto cooperatively produce a time delay for sounds entering the minuteapertures to the time at which the sounds are effective for moving thediaphragm, and the peripheries of the minute apertures in the discretefoil plate being tapered in a direction through the thickness of thefoil plate to minimize the effect of the thickness of the foil plateupon the resistance offered to passage of sound.

1. A directional microphone for head-mounted eyeglass and behind-the-ear hearing aids, comprising: a rigid metal casing of box-like configuration, the casing having front and rear portions including rigid walls with exterior and interior surfaces and the walls defining front and rear access openings into the casing, the front access opening in the wall of the front portion providing substantially unrestricted sound access into the casing, the rear access opening in the wall of the rear portion being many times larger than required to provide sound access into the casing, a vibratable diaphragm in the casing and dividing the interior of the casing into two chambers, said diaphragm being disposed between said access openings and restricting air communication across the interior of the casing and between said two chambers, the diaphragm being coupled to transducer means in one of the chambers to produce electric signals in response to vibration of the diaphragm, and a discrete plate larger than said rear access opening and constructed of thin foil-like metal in contrast to the rigid metal walls of the casing, said discrete plate overlying and obstructing the entire rear access opening in the wall of the rear portion of the casing and continuously secured around the entire periphery of the plate to the exterior surface of the casing, said discrete plate having a multiplicity of minute apertures therethrough, each of said apertures having substantially the same size as adjacent apertures and each of said apertures providing open communication from the exterior of the casing and directly and linearly through the rear access opening and into the adjoining chamber within the casing, and said multiplicity of minute apertures cumulatively defining the sole acoustic resistance coordinated with the volume of the chamber between the discrete foil plate and the diaphragm to cooperatively produce a time delay for sounds entering the minute apertures to the time at which the sounds are effective for moving the diaphragm, and the peripheries of the minute apertures in the discrete foil plate being tapered in a direction through the thickness of the foil plate to minimize the effect of the thickness of the foil plate upon the resistance offered to passage of sound. 